Abnormal or aberrant neovascularization is associated with a number of diseases and disorders, including but not limited to, cancer, inflammatory disease, macular degeneration and diabetic retinopathy (DR).
The Wnt signaling pathway plays a crucial role in neovascularization and many other associated biological processes, including retinal vessel development and the inflammation process. Mutation of Wnt signaling pathway genes Frizzled-4 (Fz4) or LRP5 leads to inhibition of retinal angiogenesis in familial exudative vitreoretinopathy (FEVR) patients, while Fz4 knockout mice exhibit incomplete retinal vascularization. Moreover, VEGF is upregulated as a result of mutational activation of Wnt signaling in colon cancer and human endothelial cells. VEGF is a potent mediator of vascular permeability and angiogenesis, and is an established therapeutic target for a number of angiogenesis associated diseases, including cancer and age related macular degeneration. A number of other angiogenic regulators are also Wnt target genes including, but not limited to, FGF18, endothelin-1, Cx43, uPAR, MMP7, and MMP3.
Among the aberrant neovascularization associated diseases, diabetic retinopathy (DR) is a very common complication of diabetes mellitus and one of the four common sight-threatening conditions in developed countries. Almost 100% of patients with type I diabetes and 60% of type II diabetic patients will develop some degrees of retinopathy in their lifetime. Approximately 10% of diabetic patients develop a severe visual handicap after 15 years of diabetes. DR is a chronic and progressive disorder, primarily affecting retinal capillaries. Breakdown of the blood-retinal barrier is a common pathological change in patients with diabetes and in streptozotocin (STZ)-induced diabetic animal models. In the early stages of DR, the retinal vascular permeability is increased without the appearance of clinical retinopathy. Retinal vascular leakage and thickening of the retina lead to diabetic macular edema (DME). In the late stages of DR, over-proliferation of capillary endothelial cells results in retinal neovascularization (NV), the abnormal formation of new vessels from preexisting capillaries in the retina and vitreous. This, in turn, leads to proliferative diabetic retinopathy (PDR). The abnormal angiogenesis can ultimately cause severe vitreous cavity bleeding and/or retinal detachment, resulting in severe vision loss.
It has been shown that multiple growth factors in the eye, such as but not limited to, VEGF, bFGF, IGF-1, and PEDF, are implicated in the pathogenesis of DR. Alterations of these growth factors and their receptors in diabetes have been identified in both experimental and clinical studies. Increased VEGF levels are at least partly responsible for retinal vascular leakage, retinal vascular hyper-permeability and retinal NV in patients with DR. VEGF therefore plays an important role in the development and pathogenesis of DR. The upregulated expression of retinal VEGF and its receptors correlates with retinal NV in OIR. Inhibition of VEGF and VEGF receptors has been shown to prevent retinal NV in diabetic and OIR animal models.
Accumulating evidence indicates that the Wnt signaling pathway not only mediates inflammation, i.e., TNF-alpha, NF-κB translocation and VEGF, but also regulates angiogenesis in the eye. Studies demonstrated that both Frizzled-4 (Fz4) and Lrp5/6 are expressed in adult murine retinal vasculature. Mutations in the Fz4 or LRP5 gene in the human lead to inhibition of normal retinal angiogenesis in familial exudative vitreoretinopathy (FEVR) patients, and Fz4 knockout (fz4−/−) mice exhibited an incomplete retinal vascularization. Meanwhile, it has been shown that seven β-catenin/TCF binding sites occur in the gene promoter for VEGF-A. Under hypoxia conditions, HIF-1α competes with TCF-4 to form a new complex with β-catenin instead of β-catenin/TCF in the HIF-1α gene promoter region. Moreover, VEGF is upregulated as a result of mutational activation of the Wnt/β-catenin signaling in colon cancer cells and in human endothelial cells. A variety of other angiogenic regulators have previously been reported as Wnt target genes including but not limited to, FGF18, endothelin-1, Cx43, uPAR, MMP7, and MMP3. Thus Wnts may regulate angiogenesis through induction of multiple angiogenic genes.
The canonical pathway is initiated when a Wnt ligand binds to a member of the Frizzled serpentine receptor family and its co-receptor LRP6 or a close relative such as LRP5. When the Wnt-induced Fz-LRP6 complex forms, LRP6 will be phosphorylated at its PPPSP motif and is then capable of binding Axin in a phosphorylation-dependent manner to the plasma membrane, thereby resulting in the inhibition of β-catenin phosphorylation and degradation. LRP6 is of critical importance in human diseases. The LRP6 cytoplasmic domain is essential for Axin binding, and its deletion in LRP6 ΔC results in a dominant negative receptor that binds Wnt but is unable to bind Axin. The LRP6 extracellular domain has auto-inhibitory activity, because its deletion in LRP6ΔN results in a constitutively activated receptor that binds Axin in the absence of Wnt ligand.
As stated herein above, retinal NV is a major pathological feature leading to vision loss in DR. VEGF is a well-known key factor in stimulating the retinal NV formation in the DR.
Therefore, there exists a great need for new and improved compositions and methods for the inhibition of the Wnt signaling pathway. Such compositions and methods would be useful in the treatment and prevention of neovascularization-associated and/or Wnt signaling pathway associated diseases, including but not limited to, inflammation, fibrosis, angiogenesis and/or tumorigenesis. The presently disclosed inventive concept(s) is directed to said compositions and methods, which overcome the disadvantages and defects of the prior art.